Method for preparing thin-walled ceramic articles of configuration

ABSTRACT

A method for preparing a hollow thin-walled ceramic product is described. Ceramic powder is plasma-sprayed onto a concave surface of a substrate having a coefficient of thermal expansion less than that of the ceramic. The coated substrate is heated to sinter the ceramic and then cooled to effect a separation of the ceramic product from the substrate.

Umted States Patent 1 [111 3,917,782

Holcombe et al. Nov. 4, 1975 METHOD FOR PREPARING THIN-WALLED 3,560,364 2/1971 Burkhardt 264/81 CERAMIC ARTICLES OF CONFIGURATION 3,568,723 9" SOWardS 3,576,932 2/1969 Biddulph 264/81 [75] Inventors: Cressie E. Holcombe; George L.

Powell, both of Oak Ridge, Tenn. FOREIGN PATENTS OR APPLICATIONS 73 Assignee; The United States of America as 654,294 12/1962 Canada 264/81 represented by the United States Energy Research and Development Primary ExaminerDonald J. Arnold Administration, Washington, DC. Assistant Examiner-John Parrish Attorney, Agent, or FirmJohn A. Horan; David S. [22] Filed. May 16, 1973 Zachry; Earl L Larcher [2]] Appl. No.: 361,000

C [57] ABSIRACT 52 US. l. 264 81; 264 60-, 264 230; 264/{31G 71/264/DI 72 A method for preparing a hollow thin-walled ceramic 51 Int. CI. 829C 5/02 B29C 13/04 Pmduct is desflibed- Ceramic Powder is P [58] Field of Search 264/56 81 71 DIG 72 sprayed onto a concave surface of a substrate having a 264/230 6 coefficient of thermal expansion less than that of the ceramic. The coated substrate is heated to sinter the [56] References Cited ceramic and then cooled to effect a separation of the UNITED STATES PATENTS ceramic productfrom the substrate.

3,467,583 9/1969 Noimer 264/81 6 Claims, No Drawings METHOD FOR PREPARING 'II-IIN-WALLED CERAMIC ARTICLES F CONFIGURATION The present invention was made in the course of, or under, a contract withthe US Atomic Energy Commis:

sion. I t

The present invention relates generally to thin-walled ceramic products of a hollow configuration and more particularly to the method of preparing such products.

Ceramic artifacts such as crucibles, crucible liners, containers for fast quenching experimentsand sintering operations,.susceptor liners, conduits or, tubulations, etc. are often required in the practice of various chemical and metallurgicalprocedures where the selected ceramic is compatible, i.e., essentially non-reactive, with the material being. worked upon, particularly at elevated temperatures under vacuum or in the presence of various oxidizing, reducing and. other reactive atmospheres.,Previoustechniquesfor preparing such arti-.

facts including slip casting and sintering, hot pressing, extrusion and simple lay-up procedures have met with some success except in the fabrication of artifacts as,

mentionedabove in thin-walled (wallthicknesses less than about 0.05 inch) configurations of densities greater than about 85 percent theoretical due to cracking and distortion of the walls caused by volume changes during temperature recycling.

It is the aim or primary goal of the present invention configured as to encompass the sprayed layer, heating the coated mandrel to sinter the ceramic coating, and then cooling the coated mandrel to cause the ceramic layer toshrinkaway or separate from. the metal substrate so as to form a freestanding, thin-walled ceramic article. i

Other and further objects of the invention willbe obvious upon an understanding of the illustrative method about to be described, or wi1l be indicated in the appended claims, and various advantages not referred to herein .will occur to,one skilled in the; art upon employ ment of the invention in practice.

D escribedigenerally,the present invention is directed to a method of preparing ceramic articles of a configuration having a surface of revolution with or without a closed end as defined respectively by a crucible or container and a tubulation or right-angled cylinder The method comprises the .steps ofplasma-spraying metal oxide or ceramic .powder in air onto aconcave surface of a mandrel to form a ceramic layer or coating of the desired thickness, heating the coatedmandrel in vacuuni to a temperature adequate tosinter and anneal the ceramic layer, cooling ,the coated mandrel to effect separation of the sintered. ceramic layer from the mandrel, and thereafter removingthe separated ceramic layer from the mandrel to provide a freestanding, thinwalled ceramic article having a wall thickness in the range of 0.005 to 0.20 inch. 7 p

In order to effect the separation of the ceramic layer from the mandrel thelatter must have a CTE substan tially lower than the ceramic material forming the plasma-sprayed layer. It has been found that suitable mandrels may be formed from materials having a mean CTEof about 5 X 10 over a temperature range of 25 to l ,000".C..since most ceramic materials have mean CTEs greater than about 8 X 10 over a correspond- I ing temperature range. Adifference in the CTE between the mandrel andv the ceramic of about 3 X 10 is necessaryto effect the-desired separation. Satisfactory results-have been achieved by forming the mandrel of tungsten, which has a melting point of 3,410C. and a CTE of4.8 X 10 (25-1,000C.)f, or an alloy formed oftungsten, 3.5 weight'percent nickel, and 1.5 weight percent iron.This alloy has a melting point of 1,500C.

and a CTE of 5.2 X 10 (25l,000C.). It may be preferableto usethe alloy in most cases due to its superior machinability over tungsten. However, if the sintering temperatures required for the ceramic'material are greater than-about 1,300C., it may be preferable to use the tungsten metal. The ceramic articles formed by the plasma-spraying may be madeof-materials such as alumina, which has a CTE of 8.5 X 10 at 25- 1,000C.;.aluminum titanate, which has a CTE of about 9.5 X 10 at 25-1,000C.;'zirc'oniumtitanate, which has a CTE of 7.9 X 10 at 251,000C.;and magne sium zirconate, which hasa CTE 'of 12 X 10 at 25-l ,000C. Each of these ceramic materialsata prespray particle size of less than 200 mesh may be readily sintered at a temperature in the range of about I,3001,800C. Other ceramic'materia'ls which may be employed in the present method are stabilized zirconium oxide, thoriumoxide, chromium oxide, and any other ceramic which can be plasma-sprayed and for which a suitable substrate mandrel can be found. Other possible mandrel materials include other refractory metals such as niobium," tantalum, and molybdenum, their alloys such as Mo-0.5 Ti and Ta-l0 W, graphite, and metals with higher CTE values if the ceramic has'the required higher CTE.

Further, to assure separation of the ceramic layer from the mandrel the plasma-spraying operation is preferably accomplished in air, since vacuum and inert atmospheres such as argon promote adhesion of the layer to the mandrel.

In order to form the layer of ceramic material the selected ceramic is comminuatedinto powder of a size less 'than 200 mesh with an average particle size of about 74 microns. This powder was sprayed by a conventional, commercially available plasma-flame spray gun onto the mandrel. The resulting ceramic layer upon completion of the spraying operation has a density of about 90 percent of theoretical density and open porosity. Upon completing the spraying operation the coated mandrel is heated in vacuum of about 10 torr or in an inert atmosphere such as argon or helium to'a temperature in the range of about l,300l,800:" C", to

sinterand anneal the ceramic layer. As the temperature is so increased the ceramic material,due to its greater CTE than the mandrel, is expanded against the mandrel so as to be held in compression by the latter This feetion of about 1 to 4 hours, the coated mandrel is cooled to room temperature. During this cool-down the ceramic layer is not distorted more than approximately 1 percent of its overall dimension as it shrinks away and separates from the mandrel due to its greater CTE. The surface of the ceramic article adjacent to the mandrel has the same surface finish as the mandrel.

The configuration of the mandrel found suitable for practicing the present invention is one that will provide a concave surface about the layer of ceramic material. The inner walls of the mandrel forming this surface or receptacle for the ceramic material may be slightly .tapered, i.e., upto about 5, and have a depth of up to about 2 inches and a diameter of up to about 2 inches. Alternatively, the receptacle may be in the form of a right-angled cylinder of a diameter and a depth limited only by the spraying capabilities of the plasma gun.

To more clearly set forth features of the present invention examples relating to the fabrication of ceramic artifacts are set'forth below.

EXAMPLE 1 Atungsten-3.5 percent nickel-1.5 percent iron alloy mandrel having a maximum inner diameter of 2.0 inches, a depth of 1.0 inch, a wall taper of 5 to the mandrel axis, andcorners that were rounded to a minimum radius of 0.05 inch was plasma-sprayed in air with aluminum titanate powder. The powder was prepared from commercially obtained aluminum and titanium oxide having an average particle size less than 25 microns. Stoichiometric proportions of the oxides were blended, pressed and sintered to form a compact. The compactwas ground into powder having an average particle size of less than 74 microns for the plasmaspraying operation. After the mandrel was sprayed with the aluminum titanate powder, the coating was sintered in a furnace gradually heated to 1,300C. over a 3-hour period at a pressure of l X torr. After the furnace cooled to room temperature, the coated crucible was removed by simply inverting the crucible. Three aluminum titanate crucibles having 0.012 i0.002-inch wall thicknesses were made in this manner.

EXAMPLE 11 One alumina crucible having a 0.012 i0.002-inch wall thickness and one magnesium zirconate crucible having a wall thickness of 0.017 $0.003 inch were prepared as in Example 1 using commercially available -200 mesh powders. This demonstrates that ceramic oxide articles with a wide range of thermal expansion coefficients can be fabricated by this method.

EXAMPLE 111 A tungsten-3.5 percent nickel-1.5 percent iron alloy mandrel having a receptacle in the shape of a rightangle cylinder 0.3785 inch in diameter by 0.5 inch deep was plasma-sprayed in air with alumina to a nominal thickness of 0.0 1 5 inch. Thickness variations of 10.005 inch resulted due to the difficulty of plasma-spraying within such a small opening. After a l-hour sinter at 1,300C. and cool-down, the alumina crucible was removed from the mandrel intact. The resulting crucible had an outer diameter of 0.375 10.002 inch and a density of about 90 percent theoretical.

It will be seen that the present invention affords a significant improvement over previously employed techniques of fabricating ceramic structures since the preswhereas previous practices of forming such structures.

with wall thicknesses less than 0.02 inch by slip casting and extrusion were limited to wall thickness to overall size ratio no less than 0.l. Further, the freestanding ceramic products produced by the subject method have a wall thickness in the range of about 0.005 to 0.200 inch and a density of at least percent theoretical.

What is claimed is:

1. A method of manufacturing a freestanding, hollow ceramic article of a concave configuration with a wall thickness in the range of 0.005 to 0.20 inch and a density of at least 90 percent theoretical, comprising ,the steps of plasma flame spraying powdered ceramic material onto a concave surface within a reusable metal substrate to form a coating of the ceramic material on said surface, said metal substrate having a coefficient of thermal expansion substantially less than that of said ceramic material, heating the coated substrate to a temperature adequate to anneal and sinter the ceramic coating with said heating simultaneously effecting the expansion of the ceramic coating against the substrate for holding the coating in compression, reducing the temperature of the'coated substrate to shrink the ceramic coating relative to the substrate to effect separation of the ceramic coating from the substrate, and thereafter removing the resulting ceramic article from the substrate. I

2. The method of manufacturing a hollow ceramic article as claimed in claim I, wherein the difference in the average coefficient of thermal expansion between the substrate and the ceramic material is at least about 3 X 10' over a temperature range of 251,000C.

3. The method of manufacturing a'hollow ceramic article as claimed in claim 2, wherein said substrate is of a metal or alloy having an average coefficient of thermal expansion less than about 5 X 10 over a temperature range of 251,000C., and wherein said c'eramic material has an average coefficient of thermal expansion greater than about 8 X 10 over a temperature range of 251,000C.

4. The method of manufacturing a hollow' ceramic article as claimed in claim 2, wherein the reusable substrate is of a metal or alloy selected from the group consisting of tungsten, a tungsten alloy containing 3.5 weight percent nickeland 1.5 weight percent iron, molybdenum, tantalum, niobium, and graphite.

5. The method of manufacturing a hollow ceramic article as claimed in claim 3, wherein said ceramic material is selected from the group consisting of aluminum titanate, alumina, zirconium titanate, magnesium zirconate, stabilized zirconium oxide, thorium oxide, and chromium oxide, and wherein the particle size of the powdered ceramic material is less than 200 mesh.

6. The method of manufacturing a hollow ceramic article as claimed in claim 5, wherein the ceramic material is plasma-sprayed onto said concave surface in air, wherein the temperature adequate to anneal andsinter the substrate is in the range of 1,300 to 1,800C., wherein duration of said heating step is in the range of 1 to 4 hours in an inert atmosphere or a vacuum at a pressure of about 10' torr, and wherein the temperature is reduced to room temperature after said heating step. 

1. A METHOD OF MANUFACTURING A FREESTANDING, HOLLOW CERAMIC ARTICLE OF A CONCAVE CONFIGURATION WITH A WALL THICKNESS IN THE RANGE OF 0.005 TO 0.20 INCH AND A DENSITY OF AT LEAST 90 PERCENT THEORTICAL COMPRISING THE STEPS OF PLASMA FLAME SPRAYING POWDERED CERAMIC MATERIAL ONTO A CONCAVE SURFACE CITHIN A REUSABLE METAL SUBSTRATE TO FORM A COATING OF THE CERAMIC MATERIAL ON SAID SURFACE, SAID METAL SUBSTRATE HAVING A COEFFICIENT OF THERMAL EXPANSION SUBSTANTIALLY LESS THAN THAT OF SAID CERAMIC MATERIAL, HEATING THE COATED SUBSTRATE TO A TEMPERATURE ADAQUATE TO ANNEAL AND SINTER THE CERAMIC COATING WITH SAID HEATING SIMULTANEOUSLY EFFECTING THE EXPANSION OF THE CERAMIC COATING AGAINST THE SUBSTRATE FOR HOLDING THE COATING IN COMPRESSION, REDUCING THE TEMPERATURE OF THE COATED SUBSTRATE TO SHRINK THE CERAMIC COATING RELATIVE TO THE SUBSTRATE TO EFFECT SEPARATION OF THE CERAMIC, COATING FROM THE SUBSTRATE, AND THEREAFTER REMOVING THE RESULTING CERAMIC ARTICLE FROM THE SUBSTRATE.
 2. The method of manufacturing a hollow ceramic article as claimed in claim 1, wherein the difference in the average coefficient of thermal expansion between the substrate and the ceramic material is at least about 3 X 10 6 over a temperature range of 25*-1,000*C.
 3. The method of manufacturing a hollow ceramic article as claimed in claim 2, wherein said substrate is of a metal or alloy having an average coefficient of thermal expansion less than about 5 X 10 6 over a temperature range of 25*-1,000*C., and wherein said ceramic material has an average coefficient of thermal expansion greater than about 8 X 10 6 over a temperature range of 25*-1,000*C.
 4. The method of manufacturing a hollow ceramic article as claimed in claim 2, wherein the reusable substrate is of a metal or alloy selected from the group consisting of tungsten, a tungsten alloy containing 3.5 weight percent nickel and 1.5 weight percent iron, molybdenum, tantalum, niobium, and graphite.
 5. The method of manufacturing a hollow ceramic article as claimed in claim 3, wherein said ceramic material is selected from the group consisting of aluminum titanate, alumina, zirconium titanate, magnesium zirconate, stabilized zirconium oxide, thorium oxide, and chromium oxide, and wherein the particle size of the powdered ceramic material is less than 200 mesh.
 6. The method of manufacturing a hollow ceramic article as claimed in claim 5, wherein the ceramic material is plasma-sprayed onto said concave surface in air, wherein the temperature adequate to anneal and sinter the substrate is in the range of 1, 300* to 1,800*C., wherein duration of said heating step is in the range of 1 to 4 hours in an inert atmosphere or a vacuum at a pressure of about 10 5 torr, and wherein the temperature is reduced to room temperature after said heating step. 